Process for the production of polymer microparticles

ABSTRACT

The object of the present invention is to provide a process for producing high-quality polymer microparticles having uniform particle size of the order of several micrometers to tens of micrometers by inverse suspension polymerization at high productivity while keeping excellent dispersion stability without causing aggregation among particles. The process is one for the production of polymer microparticles by inverse polymerization of a vinyl monomer and is characterized in that an oil-soluble oxidizing agent and a water-soluble reducing agent are used as a polymerization initiator and the oil-soluble oxidizing agent is fed after the water-soluble reducing agent is fed.

FIELD OF THE INVENTION

The present invention relates to a method for producing polymermicroparticles. More specifically, the present invention relates to amethod for producing high-quality polymer microparticles uniform inparticle size having a specific range of particle size by inversesuspension polymerization of a vinyl-based monomer at high productivity,without causing aggregation of particles and in a stable even if thescale is increased.

BACKGROUND ART

Micron-sized spherical polymer microparticles are utilized for cosmeticadditives, supports for various chemical materials, spacers, columnpackings for chromatography, light diffusion agents, porosificationagents, weight-lightening agents, antiblocking agents, surfacemodification agents for recording paper, and the like.

Among these, hydrophilic crosslinked polymer microparticles can be usedas hydrous gel microparticles, and are useful as cosmetics additives,supports, porosification agents, weight-lightening agents, and surfacemodification agents for recording paper.

Production of polymer particles by inverse suspension polymerization ofa vinyl-based monomer has conventionally been carried out. Astechnologies of producing hydrophilic crosslinked polymer particles byinverse suspension polymerization, there have been known a method inwhich a water-in-oil microdispersed droplet of a monomer is formed usinga compound having a specific HLB as a dispersing agent beforepolymerization and then the monomer is polymerized while dropping it(see Patent Document 1), a method in which inverse suspensionpolymerization is carried out in the presence of water-absorptivepolymer particles, an oil-soluble polymerization initiator and adispersing agent, and during or after the polymerization a hydrophobicvinyl-based monomer and an oil-soluble polymerization initiator areadded to perform polymerization (see Patent Document 2), a method inwhich a hydrophilic vinyl-based monomer is an inverse suspensionpolymerized in the presence of a silicone compound having at least onefunctional group in the reaction system (see Patent Document 3), and thelike.

In these conventional technologies, there are problems that thedispersion stability of polymer particles during or after polymerizationis not sufficient, the particle size of polymer particles obtained isnonuniform, and the hydrophilicity of polymer particles obtained isdegraded. In particular, when hydrophilic particles with a high degreeof crosslinking are produced while increasing the proportion of amultifunctional vinyl-based monomer used, polymerization stability issignificantly degraded, and problems such as aggregation of particles,degradation in the quality of polymer particles obtained, and areduction in productivity easily occur.

Since all the above-mentioned production methods are ones in whichpolymerization is performed by feeding a monomer emulsion continuouslyover one hour or more to a reactor heated to a high temperature of 70°C. or higher, aggregation of particles or the like easily occurs and theparticle size of the resulting polymer particles becomes irregular. Inaddition, when a large amount of a crosslinking agent such as amultifunctional vinyl-based monomer, is used, most part of unreactedcrosslinking agent becomes easy to flow out into a continuous phaseside, and when polymerization is continued in this state, particlesaggregate more and this is expected to lead to the aforementioneddeterioration in quality of polymer particles.

Furthermore, Patent Document 4 discloses an absorptive polymer particlewhich is produced by inverse suspension polymerization using a redoxpolymerization initiator for the production of a water absorptivepolymer having a specific water absorptivity, and a polymer particle isproduced by feeding tert-butyl hydroxyperoxide which is an oil-solubleoxidizing agent, and then feeding sodium bisulfite which is awater-soluble reducing agent.

According to this production method, particle size control ofmicroparticles can be performed more precisely in comparison toaforementioned conventional technologies. Since a polymerizationreaction occurs before the water-soluble reducing agent is diffusedsufficiently, this is not satisfactory as a method for producinghigh-quality particles that are uniform in particle size and have aparticle size falling within a specified range, in a stable statewithout causing, for example, aggregation of particles.

Patent Document 1: JP-A H05-222107

Patent Document 2: JP-A 2003-301019

Patent Document 3: JP-A 2003-34725

Patent Document 4: JP-A 2004-262747

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

The object of the present invention is to provide a method for theproduction of high-quality polymer microparticles uniform in particlesize having a particle size of the order of several micrometers to tensof micrometers at high productivity while keeping excellent dispersionstability without causing aggregation of particles by inverse suspensionpolymerization.

In particular, the object of the present invention is to provide aninverse suspension polymerization method in which high-quality polymermicroparticles uniform in particle size can be smoothly produced at highproductivity while keeping high polymerization stability and suspensionstability even if hydrophilic particles having a high degree ofcrosslinking are produced.

Means for Solving the Problems

The present inventors have engaged in an intensive investigation inorder to attain the above objects. The inventors thought that it isimportant to perform the polymerization at a lower temperature for ashorter time in comparison to the conventional technologies for theproduction of polymer microparticles by subjecting a vinyl-based monomerto inverse suspension polymerization, and studied the conditions. As aresult, when an oil-soluble oxidizing agent and a water-soluble reducingagent are used as a polymerization initiator, and inverse suspensionpolymerization in which the water-soluble reducing agent is fed and thenthe oil-soluble oxidizing agent is fed is carried out, it is found thathigh-quality spherical particles having a particle size of the order ofseveral micrometers to tens of micrometers and being uniform in particlesize can be produced while keeping excellent dispersion stability andpolymerization stability without causing aggregation, clumping, andadhesion to a polymerization apparatus of polymer particles during orafter polymerization.

The present invention for solving the above-mentioned problems is asfollows.

The first invention is a method for producing polymer microparticles byinverse suspension polymerization of a vinyl-based monomer, and ischaracterized in that an oil-soluble oxidizing agent and a water-solublereducing agent are used as a polymerization initiator, and that theoil-soluble oxidizing agent is fed after the water-soluble reducingagent is fed.

The second invention is a method for producing polymer microparticlesaccording to the first invention, wherein the oil-soluble oxidizingagent is fed over a time range from 20 seconds to 120 seconds.

The third invention is a method for producing polymer microparticlesaccording to the first or second inventions, wherein the oil-solubleoxidizing agent is fed to a reactor through a feed port located belowthe reaction liquid level.

The fourth invention is a method for producing polymer microparticlesaccording to any one of the first to third inventions, wherein amacromonomer having a radically polymerizable unsaturated group at anend of a polymer derived from a vinyl-based monomer is used as adispersion stabilizer.

The fifth invention is a method for producing polymer microparticlesaccording to any one of the first to fourth inventions, wherein thepolymer microparticles produced by inverse suspension polymerization arehaving a crosslink density of 0.5% or more by mol.

The sixth invention is a method for producing polymer microparticlesaccording to any one of the first to fifth inventions, wherein thepolymer microparticles produced by inverse suspension polymerization arepolymer microparticles which have an average particle size in asaturated water-swollen state of 2 to 100 μm, and a content ratio ofparticles having a particle size of 150 μm or larger in a saturatedwater-swollen state of 1.0% or less by weight.

The seventh invention is a method for producing polymer microparticlesaccording to any one of the first to fifth inventions, wherein thepolymer microparticles produced by inverse suspension polymerization arepolymer microparticles which have a water absorption ratio of 5 to 50times, an average particle size in a saturated water-swollen state of 5to 70 μm, and a content ratio of particles having a particle size of 150μm or larger in a saturated water-swollen state of 0.3% or less byweight.

Effect of the Invention

According to the production method of the present invention,high-quality spherical hydrophilic polymer microparticles remarkablyhigher uniform in particle size than those by conventional technologiescan be produced at high productivity while keeping high dispersionstability and polymerization stability without causing aggregation,clumping and adhesion to a polymerization apparatus of particles duringor after polymerization. Moreover, according to the production method ofthe present invention, even if hydrophilic particles having a highdegree of crosslinking are produced using a large amount of amultifunctional vinyl-based monomer, high-quality hydrophiliccrosslinked polymer microparticles uniform in particle size can beproduced at high productivity without causing aggregation, clumping andadhesion to a polymerization apparatus of particles.

Furthermore, when the production is carried out under conditions of anincreased scale for increasing the productivity, the production methodof the present invention can lead to high-quality polymermicroparticles.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a microscope photograph of polymer microparticles T-1 (afterpolymerization, in-oil dispersion);

FIG. 2 is a microscope photograph of polymer microparticles T-1 (afterpolymerization, in-water dispersion); and

FIG. 3 is a diagram showing an apparatus for the measurement of thewater absorption ratio of polymer microparticles.

EXPLANATION OF THE REFERENCE NUMBERS

-   -   1: burette, 2: pinch cock, 3: silicone tube, 4:        polytetrafluoroethylene tube, 5: funnel, 6: sample (polymer        microparticles), 7: filter paper for fixing sample (polymer        microparticles), 8: supporting cylinder, 9: adhesive tape, 10:        filter paper for device, 11: lid, 12: ion exchange water

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiments of the present invention are described indetail.

The “inverse suspension polymerization of a vinyl-based monomer”according to the present invention means a polymerization using an oilphase as a dispersion medium and an aqueous phase as a dispersoid. Ingeneral, in the case where polymerization is performed using ahydrophilic vinyl-based monomer, particles are produced by w/o typeinverse suspension polymerization in which an aqueous phase (an aqueoussolution of the hydrophilic vinyl-based monomer) is suspended in theform of droplets in an oil phase (a dispersion medium composed of ahydrophobic organic solvent).

The inverse suspension polymerization of a vinyl-based monomer accordingto the present invention is one in which the vinyl-based monomer issubjected to inverse suspension polymerization using an oil-solubleoxidizing agent and a water-soluble reducing agent as a polymerizationinitiator in the presence of a dispersion stabilizer.

The preferable method in the present invention is one which comprisescharging a monomer mixture prepared in advance by stirring and uniformlydissolving a vinyl-based monomer (and its neutralized product) and waterto a reaction liquid in which an oil Phase prepared using a dispersionstabilizer and a hydrophobic organic solvent has been charged, thenfeeding a water-soluble reducing agent, and subsequently feeding anoil-soluble oxidizing agent to initialize the polymerization.

The vinyl-based monomer for the inverse suspension polymerizationaccording to the present invention is not particularly limited so longas it is a radically polymerizable vinyl-based monomer. For example, ahydrophilic monomer having a hydrophilic group such as a carboxyl group,an amino group, a phosphoric acid group, a sulfonic acid group, an amidegroup, a hydroxyl group, a quaternary ammonium group or the like can beused. Among these, when a monomer having a carboxyl group, a sulfonicgroup or an amide group is used, polymer microparticles high inhydrophilicity and excellent in water absorption capacity andwater-retaining property can be obtained, being preferable.

Specific examples of the hydrophilic vinyl-based monomer include avinyl-based monomer having a carboxyl group or its (partially)alkali-neutralized product, such as (meth)acrylic acid, crotonic acid,itaconic acid, maleic acid, fumaric acid, monobutyl itaconate, monobutylmaleate and cyclohexanedicarboxylic acid; a vinyl-based monomer havingan amino group or its (partially) acid-neutralized product or its(partially) quaternary product, such asN,N-dimethylaminoethyl(meth)acrylate,N,N-diethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate, andN,N-dimethylaminopropyl(meth)acrylamide; N-vinylpyrrolidone,acryloylmorpholine; a vinyl-based monomer having a phosphoric acidgroup, or its (partially) acid-neutralized product, such as acidphosphoxyethyl methacrylate, acid phosphoxypropyl methacrylate, and3-chloro-2-acid phosphoxypropyl methacrylate; a vinyl-based monomerhaving a sulfonic acid group or phosphonic acid group, or its(partially) alkali-neutralized product, such as2-(meth)acrylamide-2-methylpropanesulfonic acid,2-sulfoethyl(meth)acrylate, 2-(meth)acryloylethanesulfonic acid,allylsulfonic acid, styrene sulfonic acid, vinylsulfonic acid,allylphosphonic acid, and vinylphosphonic acid; a nonionic hydrophilicmonomer such as (meth)acrylamide, N,N-dimethyl acrylamide, N-isopropylacrylamide, N-methylol(meth)acrylamide, N-alkoxymethyl(meth)acrylamide,(meth)acrylonitrile, hydroxyethyl(meth)acrylate,hydroxypropyl(meth)acrylate and the like. These compounds may be usedsingly or in combination of two or more types thereof.

Using a compound selected from (meth)acrylic acid, (meth)acrylamide and2-acrylamide-2-methylpropanesulfonic acid singly or in combination oftwo or more types for the inverse suspension polymerization ispreferable from the viewpoint that polymerizability is excellent andresulting particles are excellent in water absorption property. Theparticularly preferred is (meth)acrylic acid.

In the present invention, a multifunctional vinyl-based monomer havingtwo or more radically polymerizable unsaturated groups may be used as avinyl-based monomer together with at least one of the above-mentionedmonofunctional hydrophilic vinyl-based monomer for the inversesuspension polymerization.

Therefore, the “vinyl-based monomer” according to the present inventionis a general term for the monofunctional vinyl-based monomer and themultifunctional vinyl-based monomer.

The multifunctional vinyl-based monomer is not particularly limited solong as it has two or more groups radically polymerizable with theabove-mentioned hydrophilic vinyl-based monomer, and specific examplethereof includes a di- or tri-(meth)acrylate of a polyol, such aspolyethylene glycol di(meth)acrylate, polypropylene glycoldi(meth)acrylate, glycerin tri(meth)acrylate, trimethylolpropanetri(meth)acrylate, and a tri(meth)acrylate of a modifiedtrimethylolpropane ethylene oxide; a bisamide such asmethylenebis(meth)acrylamide; divinyl benzene, allyl(meth)acrylate, andthe like. These compounds may be used singly or in combination of two ormore types thereof.

Among these, polyethylene glycol diacrylate and methylenebis(meth)acrylamide are suitably used as the multifunctional vinyl-basedmonomer because it excels in solubility in a mixed liquid of water and abase-forming hydrophilic vinyl-based monomer and it is advantageous inbeing used in an amount increased for obtaining a high degree ofcrosslinking. The particular preferred is polyethylene glycoldi(meth)acrylate.

The amount of the multifunctional vinyl-based monomer to be used dependson the type of the vinyl-based monomer to be used and the intendedapplication of resulting particles. When the polymer particles arerequired to have crosslinked characteristics, the amount thereof ispreferably in the range from 0.1 to 100 mol, more preferably from 0.2 to50 mol, and further preferably from 0.5 to 10 mol based on 100 mol ofthe total amount of the monofunctional vinyl-based monomer to be used.

Examples of the hydrophobic organic solvent that forms an oil phase(dispersion medium) in the inverse suspension polymerization accordingto the present invention include an aliphatic hydrocarbon solvent having6 or more carbon atoms; an aromatic hydrocarbon solvent such as benzenetoluene, xylene and ethyl benzene; a silicone-based solvent such asoctamethylcyclotetrasiloxane, and the like. In particular, hexane,cyclohexane, and n-heptane are suitably used because the solubilities ofvinyl-based monomer and water in the solvent are small and they can beremoved easily after polymerization.

In the inverse suspension polymerization according to the presentinvention, a hydrophilic vinyl-based monomer (and a neutralized saltthereof) is preferably dissolved in water to form an aqueous solutionand then is added to the polymerization system. The concentration of thehydrophilic vinyl-based monomer in the aqueous solution in which thehydrophilic vinyl-based monomer is dissolved is preferably in the rangefrom 5% to 80% by weight, and particularly from 20% to 60% by weightfrom the viewpoint that the inverse suspension polymerization proceedssmoothly and the productivity is good.

In the case where the hydrophilic vinyl-based monomer for the inversesuspension polymerization is a vinyl-based monomer having an acidicgroup such as a carboxyl group and a sulfonic acid group, when thehydrophilic vinyl-based monomer is added to water and the acidic groupin the vinyl-based monomer is neutralized with an alkali aqueoussolution such as aqueous ammonia, an aqueous sodium hydroxide solution,and an aqueous potassium hydroxide solution, an aqueous solution can beprepared in which the vinyl-based monomer is dissolved sufficiently.

In the producing method of the present invention, a dispersionstabilizer is an essential component.

Specific examples of the dispersion stabilizer include a macromonomertype dispersion stabilizer, and a nonionic surfactant such as a sorbitanfatty acid ester, a polyglycerol fatty acid ester, a sucrose fatty acidester, sorbitol fatty acid ester and a polyoxyethylene alkyl ether.

Among these, a macromonomer type dispersion stabilizer is preferable.The macromonomer type dispersion stabilizer is a vinyl-basedmonomer-derived polymer having, at an end thereof, a radicallypolymerizable unsaturated group.

Moreover, it is preferable to use a relatively high hydrophobic nonionicsurfactant having an HLB of 3 to 8, such as sorbitan monooleate andsorbitan monopalmitate, together with a macromonomer type dispersionstabilizer. These surfactants may be used singly or in combination oftwo or more types thereof.

The preferable macromonomer as the above-mentioned macromonomer typedispersion stabilizer are a macromonomer having an α-substituted vinylgroup represented by the following formula (1), at an end of a polymerobtained by radical polymerization of a vinyl-based monomer at atemperature range of 150° C. to 350° C., and/or a macromonomer having a(meth)acryloyl group at an end of a polymer derived from a vinyl-basedmonomer.H₂C═C(X)—  (1)(In the Formula, X is a Monovalent Polar Group.)

These macromonomers are excellent as a dispersion stabilizer andpreferable. The weight average molecular weight of the macromonomer ispreferably in the range from 1,000 to 30,000. The macromonomerpreferably has both structural units derived from a hydrophilicvinyl-based monomer and a hydrophobic vinyl-based monomer. Thestructural unit derived from the hydrophobic vinyl-based monomer ispreferably a structural unit derived from a (meth)acrylic acid alkylester having 8 or more carbon atoms, and the structural unit derivedfrom the hydrophilic vinyl-based monomer is preferably a structural unitderived from a vinyl-based monomers having a carboxyl group.

In particular, when the hydrophilic particles are produced by inversesuspension polymerization of a vinyl-based monomer using a macromonomertype dispersion stabilizer, it is preferable to use a multifunctionalvinyl-based monomer together with a monofunctional compound. Therebyhydrophilic particles having improved strength and shape retainabilitycan be obtained.

The dispersion stabilizer is preferably added to a polymerization systemafter being dissolved or uniformly dispersed in a hydrophobic organicsolvent that is a dispersion medium (oil phase).

The amount of the dispersion stabilizer to be used is preferably in therange from 0.1 to 50 parts by weight, more preferably from 0.2 to 20parts by weight, and further preferably from 0.5 to 10 parts by weightbased on 100 parts by weight of the total amount of the vinyl-basedmonomer in order to obtain hydrophilic polymer microparticles uniform inparticle size while maintaining excellent dispersion stability. If theamount of the dispersion stabilizer used is too small, the dispersionstabilities of the vinyl-based monomer and formed polymer microparticlesin the polymerization system becomes poor, and the formed particleseasily aggregate, precipitate, and have variation in particle size. Onthe other hand, if the amount of the dispersion stabilizer used is toolarge, the amount of the microparticles (having size of 1 μm or smaller)by-produced may be increased.

In the inverse suspension polymerization according to the presentinvention, it is preferable to carry out the polymerization so that theweight ratio of an oil phase (dispersion medium) to an aqueous phase(dispersoid) in the polymerization system may become from 99:1 to 20:80,and particularly 95:5 to 30:70, from the viewpoint that productivity,dispersion stability during polymerization, and control of particle sizeof the polymer microparticles can be satisfied at the same time.

The inverse suspension polymerization according to the present inventionis preferably carried out under stirring and the reaction is preferablyperformed in a reaction vessel equipped with a stirring blade or abaffle. As the stirring blade, an anchor blade and a paddle blade arepreferable, and a paddle blade is particularly preferred. Generally,suspension polymerization is influenced by stirring power. If thestirring power is excessively low, polymer microparticles having adesired particle size cannot be obtained or it is impossible to inhibitan aqueous solution of monomers from merging, and, as a result, problemsmay arise, such as that a perfectly spherical microparticle can not beobtained or that many aggregated particles are formed.

In the present invention, the stirring power per unit volume in thereaction vessel is preferably 0.5 kw/m³ or higher, and particularly 1.0kw/m³ or higher.

In the inverse polymerization according to the present invention, aredox type initiator including an oil-soluble oxidizing agent and awater-soluble reducing agent is used as a polymerization initiator. Aredox reaction makes it possible to proceed a polymerization initiationat a low temperature, increase the concentration of a vinyl-basedmonomer in a polymerization reaction liquid and increase thepolymerization rate. Therefore, the productivity can be improved and themolecular weight of a polymer formed can be higher.

As mentioned above, a hydrophobic organic solvent is used as acontinuous phase (oil phase) in which a dispersion stabilizer isdissolved or dispersed in the inverse suspension polymerization. Theoil-soluble oxidizing agent means an oxidizing agent which dissolves insuch a continuous phase.

The oil-soluble oxidizing agent according to the present invention ispreferably a compound having an octanol/water partition coefficient(logPow) provided in Japanese Industrial Standards Z7260-107 or OECDTEST Guideline 107 of preferably −1.4 or more, more preferably 0.0 ormore, and further preferably 1.0 or more.

Specific example thereof includes an organic peroxide such as tert-butylhydroperoxide (logPow=1.3), di-tert-butyl hydroperoxide, tert-hexylhydroperoxide, di-tert-amyl hydroperoxide, cumene hydroperoxide(logPow=2.2), dicumyl peroxide (logPow=5.5), tert-butyl cumyl peroxide,tert-butyl peroxy pivalate, benzoyl peroxide (logPow=3.5), and lauroylperoxide. Among these, tert-butyl hydroperoxide and cumene hydroperoxideare preferable. The particular preferred is cumene hydroperoxide.

Reducing agents known as a reducing agent to be used as a redox typepolymerization initiator can be used as the water-soluble reducingagent. Among these, sodium sulfite, sodium hydrogensulfite, and sodiumhydrosulfite are preferable. Particularly preferred is sodiumhydrosulfite. Since the water-soluble reducing agent as such isdeactivated gradually through their contact with air, it is preferableto dissolve the agent in water several minutes before a desired time ofstarting polymerization and then add it.

It is necessary for the oil-soluble oxidizing agent and thewater-soluble reducing agent that the water-soluble reducing agentshould be fed to a reactor first and then the oil-soluble oxidizingagent should be fed to the reactor. It is preferable that after thewater-soluble reducing agent is water-solubilized to charge into thereactor, the oil-soluble oxidizing agent is fed within 0.5 to 15 minutesand preferably within 1 to 5 minutes to perform the polymerization.

The whole amount of the oil-soluble oxidizing agent is fed to thereactor over a time of preferably 20 to 120 seconds, and particularly 20to 60 seconds.

It is undesirable that the feed time of the oil-soluble oxidizing agentis shorter than 20 seconds because, if so, diffusion of the oxidizingagent may fail to catch up with the feed of the oxidizing agent andcause local generation of radicals, which may easily result in troublessuch as generation of aggregates. On the other hand, if it is longerthan 120 seconds, the oxidizing agent may be partially remainedunreacted in the system due to consumption of the reducing agent causedby the decomposition thereof occurring in another mechanism. It isundesirable that the oxidizing agent remains unreacted because, this maycause of troubles, such as generation of aggregates during the followingazeotropic dehydration step, the drying step, and the like.

There is no particular limitation on the feed time of a water-solublereducing agent, and it is preferable to feed it within 15 minutesbecause the reducing agent is generally easily decomposed due to theircontact with the air or the like.

The oil-soluble oxidizing agent is preferably fed to a reactor through afeed port located below the reaction liquid level. Generally, a feedport for a polymerization catalyst is provided at an upper portion of areactor and a polymerization catalyst is fed at one time or continuouslythrough this port to the reaction liquid level. In the presentinvention, a method of feeding a polymerization catalyst into a reactionliquid through a pipe connected to a side wall of a reactor ispreferable from the viewpoint of uniform diffusion of the catalyst.

There is no particular limitation on the position of the feed port solong as the port is located at a position which is always immersed in areaction liquid. The feed port is preferably located at a positionwithin ±1 m in terms of the vertical height from the upper end or thelower end of a stirring blade, and is more preferably located at aposition within ±50 cm.

Examples of the method for feeding the oil-soluble oxidizing agentinclude a method for feeding it through a pipe leading to a feed portlocated in a portion below the reaction liquid level, by using a pump orgas pressure of an inert gas such as nitrogen.

The amount of the polymerization initiator to be used may be adjustedaccording to the types of the vinyl-based monomer and the particle sizeand molecular weight of the resultant polymer microparticles. The amountof the oil-soluble oxidizing agent is in the range from 0.001 to 0.15mol, and particularly from 0.003 to 0.07 mol based on 100 mol of thetotal of the vinyl-based monomer.

Additionally, the ratio of the oil-soluble oxidizing agent and thewater-soluble reducing agent is not particularly limited. The molarratio of the oil-soluble oxidizing agent to the water-soluble reducingagent is preferably 1.0 to 0.25-15.0, and particularly 1.0 to 1.0-10.0.

If the ratio is outside that range, the unfavorable may be occurred.Example thereof includes a generation of aggregates caused by loweringof the reaction rate of monomers, shortening of the chain of a polymerconstituting particle, remaining of a catalyst after the completion ofpolymerization; and the like.

In the inverse suspension polymerization according to the presentinvention, the temperature of the reaction liquid at the time ofstarting the polymerization is preferably in the range from 0° C. to 40°C., more preferably from 5° C. to 30° C., and particularly from 10° C.to 25° C. If the reaction start temperature is lower than 0° C.,freezing of a polymerization facility or a reaction solution becomes aproblem and a large cost is required for cooling. On the other hand, ifthe reaction start temperature exceeds 40° C., it is necessary, from asafety aspect, to reduce the amount of monomers to be fed, resulting ina large production cost.

In the production method of the present invention, the average particlesize of the resultant polymer microparticles is preferably in the rangefrom 2 to 150 μm, more preferably from 2 to 100 μm, and furtherpreferably from 5 to 70 μm. If the average particle size is smaller than2 μm, the slipping property or blocking preventing function may beinsufficient. If it exceeds 150 μm, an problem such as unfavorableappearance, deterioration of touch feeling and lowering of the strengthafter incorporating materials may be occurred. When the size of thepolymer microparticles becomes smaller, the stabilizing effect of thedispersion stabilizer comes to be needed more because the interfacialarea between a continuous phase and a dispersed phase becomes larger.

As to the size of the polymer microparticles, the size under a conditionwhere the particles are used becomes important. When the polymermicroparticles are used as water-swollen particles, it is preferablethat the size when the particles are swollen with water be within theabove-mentioned range.

The polymer microparticles are preferably crosslinked. As mentionedabove, the crosslinking structure of a polymer constituting themicroparticles is based on copolymerization of a multifunctionalvinyl-based monomer.

It is also possible to adjust the degree of crosslinking by reacting acrosslinking agent after polymerizing a vinyl-based monomer having afunctional group by inverse suspension polymerization. For example,there is a method in which polymer microparticle obtained using amonomer having a carboxyl group are subjected to crosslinking withethylene glycol diglycidyl ether.

Alternatively, a polymer can be crosslinked by a known method such asionic bond type crosslinking via a multivalent metal ion and covalentbond type crosslinking in which crosslinking is achieved by applicationof radiation.

When the polymer microparticles according to the above-mentionedcrosslinking method are having a crosslink density of 0.5% or more bymol, the particle can exert its characteristics in various applicationsmentioned above. Therefore, it is preferable that the polymermicroparticles have a crosslink density of 0.5% or more by mol.

After forming a dispersion liquid of polymer microparticles by inversesuspension polymerization according to the present invention, a drypowder of the polymer microparticles can be obtained using a knownmethod. A method of obtaining a dry powder by heating the dispersionliquid as it is and then removing volatile components under a reducedpressure reduction, and a method comprising removing a dispersionstabilizer, unreacted monomers and the like by performing solid-liquidseparation by filtration or centrifugal separation, and washing, andthen performing drying, are selected. To perform a washing step isdesirable because the primary dispersion property of the microparticlesafter drying increases.

Additionally, it is desirable to remove water before drying byazeotropic distillation or the like since the dispersed phase containswater. When the water is removed beforehand, it is possible to preventparticles from fusing at the time of drying and, as a result, theprimary dispersion property of the particles after drying increases.

According to the present invention, it is possible to smoothly producepolymer particles having an average particle size in a saturatedwater-swollen state of 2 to 100 μm and a content ratio of particleshaving a particle size of 150 μm or larger in a saturated water-swollenstate of 1.0% or less by weight. Such polymer particles can demonstrateits characteristics remarkably in various applications.

Furthermore, it is also possible to produce polymer particles having awater absorption ratio of 5 to 50 times, an average particle size in asaturated water-swollen state of 2 to 100 μm, and a content ratio ofparticles having a particle size of 150 μm or larger in a saturatedwater-swollen state of 0.3% or less by weight. The polymer will becomepolymer particles that lead to excellent characteristics in variousapplications.

It is noted that the water absorption ratio of the polymermicroparticles, the average particle size in a state where the particlesare saturated and swollen with water, and the content of a particlesaturated and swollen with water having a particle size of 150 μm orlarger in the present specification are values measured or determined bythe methods described in the following Example section.

EXAMPLES

Hereinafter, the present invention is described in detail usingExamples. In the following description, “part” means part by weight and“%” means % by weight.

Production Example 1 Production of Macromonomer Compositions UM-1 andUM-1HP

The temperature of an oil jacket of a 1,000-mL pressuring stirringvessel type reactor with the oil jacket was kept at 240° C.

A monomer mixture liquid prepared in proportions of 75.0 parts of laurylmethacrylate (hereinafter referred to as “LMA”) and 25.0 parts ofacrylic acid (hereinafter referred to as “AA”) as a monomer, 10.0 partsof methyl ethyl ketone (hereinafter referred to as “MEK”) as apolymerization solvent, and 0.45 part of di-tert-butyl peroxide(hereinafter referred to as “DTBP”) as a polymerization initiator wascharged into a tank for starting material.

Feed of the monomer mixture liquid in the tank for starting material toa reactor was started, and the feed of the monomer mixture liquid andextraction of a reaction mixture liquid were carried out so that theweight of the contents within the reactor would be 580 g and the averageresidence time would be 12 minutes. The temperature in the reactor andthe pressure in the reactor were adjusted to 235° C. and 1.1 MPa,respectively. The reaction mixture liquid extracted from the reactor wasdepressurized to 20 kPa and continuously fed to a thin-film evaporatormaintained at 250° C. Thereby a macromonomer composition from which amonomer, a solvent and the like was distilled was discharged. Themonomer, the solvent and the like that were distilled were cooled with acondenser and collected as a distillate.

A time when 60 minutes had elapsed from a time when the temperature inthe reactor had become stable at 235° C. after the start of the feed ofthe monomer mixture liquid was defined as a collection starting point,from which the reaction was continued for 48 minutes and then amacromonomer composition UM-1 was collected. During this period, 2.34 kgof the monomer mixture liquid was fed to the reactor, and 1.92 kg of themacromonomer composition was collected from the thin-film evaporator.Moreover, 0.39 kg of the distillate was collected in a distillationtank.

The distillate was analyzed by gas chromatography, and it was found that100 parts by weight of the distillate contained 31.1 parts of LMA, 16.4parts of AA, and 52.5 parts of the solvent and others.

From the amount and the composition of the monomer mixture liquid fed,the amount of the macromonomer composition collected, and the amount andthe composition of distillate collected, the reaction rate of themonomer was calculated to be 90.2%, and the constitutional monomercomposition ratio of the macromonomer composition UM-1 was calculated tobe LMA to AA=76.0 to 24.0 (weight ratio).

The molecular weight of the macromonomer composition UM-1 was measuredby gel permeation Chromatography (hereinafter referred to as “GPC”)using tetrahydrofuran as an eluate and the polystyrene-equivalent weightaverage molecular weight (hereinafter referred to as “Mw”) and thepolystyrene-equivalent number average molecular weight (hereinafterreferred to as “Mn”) were 3,800 and 1,800, respectively. Additionally,the concentration of terminal ethylenically unsaturated bond in themacromonomer composition was determined through ¹H-NMR measurement ofthe macromonomer composition. From the concentration of terminalethylenically unsaturated bond obtained by ¹H-NMR measurement, Mnobtained by GPC, and the constitutional monomer ratio, the introductionratio of the terminal ethylenically unsaturated bond (hereinafterreferred to as “F value”) of the macromonomer composition UM-1 wascalculated to be 97%.

The produced macromonomer composition UM-1 was dissolved by heating inan appropriate amount of n-heptane, and then n-heptane was added so thatthe solid concentration would become 30.0%±0.5%. Thus, n-heptanesolution UM-1HP containing the macromonomer composition UM-1 wasproduced. The solid concentration was measured from a heat-calculatedfraction after heating at 150° C. for one hour.

As to starting materials including a monomer, a polymerization solvent,a polymerization initiator and the like, commercially availableindustrial products were used as received without performing anytreatment, such as purification.

Example 1 Production of Polymer Microparticles T-1

For a polymerization reaction was used a reactor having a capacity of250 liters, equipped with a stirring mechanism consisting of a pitchedpaddle stirring blade and two vertical baffles and further equipped witha thermometer, a reflux condenser, and a nitrogen introduction tube. Thenitrogen introduction tube is separated into two branches at the outsideof the reactor and has a configuration capable of feeding nitrogenthrough one branch and a polymerization catalyst through the other byusing a pump. The nitrogen introduction tube is connected to a wall ofthe reactor at an almost the same height as the upper end of thestirring blade. Charging was conducted so that the whole volume of thereaction liquid would become 220 liters. The details are as follows.

A reactor was charged with 4.7 parts (1.4 parts in terms of pure contentof UM-1) of the UM-1HP produced in Production Example 1 and 2.0 parts ofsorbitan monooleate (“REODOL AO-10” manufactured by KAO Corp.) as adispersion stabilizer and 400.3 parts of n-heptane as a polymerizationsolvent, which were stirred and mixed while the solution was kept at atemperature of 40° C., so that an oil phase was prepared. The oil phasewas stirred at 40° C. for 30 minutes and then was cooled to 20° C.

On the other hand, into another container were charged 100.0 parts ofAA, 13.0 parts (equivalent to 2.2 mol % relative to a monofunctionalmonomer) of polyethylene glycol diacrylate (“ARONIX M-243” manufacturedby TOAGOSEI CO., LTD., average molecular weight 425), and 95.0 parts ofion exchange water, which were stirred to be dissolved uniformly.Furthermore, while the mixed liquid was cooled so that the temperaturethereof would be kept at 40° C. or lower, 70.8 parts of a 25% aqueousammonia solution was added slowly to neutralize the mixed liquid. Thus,a monomer mixed liquid was obtained.

After setting the stirring revolution speed to be 130 rpm, the obtainedmonomer mixed liquid was charged into the reactor, so that a dispersionliquid was prepared in which the monomer mixed liquid was dispersed inthe oil phase. During this operation, the stirring power was measured tobe 1.36 kW/m³. The temperature in the reactor was kept at 20° C. andnitrogen was blown into the dispersion liquid to remove oxygen in thereactor. At a time when 1 hour and 40 minutes had passed since thecharging of the monomer mixture, an aqueous solution of 0.045 part ofsodium (Na) hydrosulfite and 0.72 part of ion exchange water was addedthrough an addition port mounted at the upper portion of the reactor.Three minutes later, a solution prepared by diluting 0.039 part of“Percumyl H80” manufactured by NOF Corp. (a 80% solution of cumenehydroperoxide) with 3.1 parts of n-heptane was fed with a pump throughthe nitrogen introduction tube. The feed was performed in 30 seconds.The temperature in the reactor increased immediately after the start ofthe feed, and this confirmed that polymerization was initiated. Theincreased internal temperature reached the peak in about two minutes,and the temperature was 66.0° C. Then, the reaction liquid was cooled toroom temperature to obtain an in-oil dispersion liquid of polymermicroparticles T-1.

When the in-oil dispersion liquid of T-1 was discharged from thereactor, the dispersion liquid was filtered using a filter having a meshopening of 75 μm. Filterability was good and no appreciable attachmentwas found on the filter after the filtration. When the attachment of aresin to the inner wall surface of the reactor was checked after thedischarge, the resin was slightly found at the vicinity of the reactionliquid level during the polymerization and it was confirmed that thepolymer microparticles T-1 could be produced stably.

When a part of the in-oil dispersion liquid of T-1 was sampled and wasobserved with a digital microscope (“KH-3000” manufactured by HIROX Co.,Ltd.) at a magnification of 420, spherical microparticles having adistribution centering approximately 10 to 20 μm were observed. Thephotograph thereof is shown in FIG. 1. No aggregated particles composedof particles united together were observed.

For a sample prepared by drying the in-oil dispersion liquid at 110° C.for one hour, the water absorption ratio (see the polymer microparticlesanalysis condition (2) below) was measured to be 20.4. When the driedsample was dispersed in an excessive amount of ion exchange water to besaturated and swollen and was observed at a magnification of 420,spherical microparticles having a distribution centering approximately30 to 40 μm were observed. The photograph thereof is shown in FIG. 2.

For the water-saturated-swollen particles T-1, particle sizedistribution measurement (see the polymer microparticles analysiscondition (3) below) was performed using a laser diffraction scatteringtype particle size distribution analyzer. The obtained particle sizedistribution had a single peak, and the water-saturated-swollen particlesize was 38.9 μm. It was confirmed that the polymer microparticles T-1had a water absorption capacity, kept spherical shape also when it wasswollen due to water absorption, and was primarily dispersed in water.Moreover, for a sample prepared by heating the T-1 in an oil bath,thereby azeotropically distilling water and heptane contained in theparticle to remove water to a dehydration degree of 95%, followed byremoval of the solvent and powdering, the amount of a wet sieve residue(see the polymer microparticles analysis condition (4) below) wasmeasured to be 0.004%. The polymer microparticles T-1 were confirmed tocontain no coarse particles greater than 150 μm also when they weresaturated and swollen with water after the azeotropic dehydration.

In Table 1, “Condition of attachment to wall” is a result of visualobservation of a resin attachment to the inner wall surface of thereactor after the completion of the reaction; “◯” indicates that therewas almost no attachment, “Δ” indicates that there was attachment at apart corresponding to the reaction liquid level, and “X” indicates thatthere was remarkable attachment on the whole wall.

“Polymerization slurry filterability” is a result of observation of thefilterability performed when a resulting dispersion liquid was filteredwith a filter having a mesh opening of 75 μm; “◯” indicates thatclogging of the filter did not occur, “Δ” indicates that cloggingoccurred once or twice, and “X” indicates that filtration at 75 μmfailed.

“Appearance of wet sieve residue after azeotropic dehydration” is aresult obtained by observing the wet sieve residue of a powdered sampleprepared by dehydration, and aggregates means that there was anaggregate composed of aggregated particles.

Example 2 Production of Polymer Microparticles T-2

Production was conducted in the same manner as that in Example 1, exceptfor setting the feeding time of the oil-soluble oxidizing agent“Percumyl H80” to 10 seconds. The results are shown in Table 1.

Example 3 Production of Polymer Microparticles T-3

Production was conducted in the same manner as that in Example 1, exceptfor setting the feeding time of the oil-soluble oxidizing agent“Percumyl H80” to 120 seconds. The results are shown in Table 1.

Example 4 Production of Polymer Microparticles T-4

Production was conducted in the same manner as that in Example 1, exceptfor setting the feeding time of the oil-soluble oxidizing agent“Percumyl H80” to 180 seconds. The results are shown in Table 1.

Example 5 Production of Polymer Microparticles T-5

Production was conducted in the same manner as that in Example 1, exceptthat an aqueous solution of sodium hydrosulfite was added through theaddition port mounted at the upper portion of the reactor at a time when1 hour and 40 minutes have passed since the charge of a monomer mixture,and that three minutes later a n-heptane solution containing “PercumylH80” was fed in 30 seconds using a pump through another port mounted atthe upper portion of the reactor. The results are shown in Table 1.

Example 6 Production of Polymer Microparticles T-6

Production was conducted in the same manner as that in Example 1, exceptthat 3.4 parts of sorbitan monooleate was used instead of 4.7 parts ofUM-1HP (pure content 1.4 parts) and 2.0 parts of sorbitan monooleate,and that 403.6 parts of n-heptane was used as a polymerization solvent.The results are shown in Table 1.

Example 7 Production of Polymer Microparticles T-7

Production was conducted in the same manner as that in Example 1, exceptfor using “Perbutyl H69” (PBH) manufactured by NOF Corp. which was a 69%solution of tert-butyl hydroperoxide as an oil-soluble oxidizing agentinstead of “Percumyl H80”. The results are shown in Table 1.

Comparative Example 1 Production of Polymer Microparticles T-8

Into a 500-ml beaker were charged 100.0 g of Acrylic acid, 13.0 gs(equivalent to 2.2 mol % relative to a monofunctional monomer) ofpolyethylene glycol diacrylate (“ARONIX M-243” manufactured by TOAGOSEICO., LTD., average molecular weight 425), and 95.0 g of ion exchangewater, which were stirred to be dissolved uniformly. Furthermore, whilethe mixed liquid was cooled so that the temperature thereof would bekept at 40° C. or lower, 70.8 g of a 25% aqueous ammonia solution wasadded slowly to neutralize the mixed liquid. Thus, a monomer mixedliquid was obtained. Furthermore, an aqueous solution of 0.33 g ofammonium persulfate salt and 4.9 g of ion exchange water was added tothe monomer mixed liquid and mixed uniformly.

Subsequently, 5 g of fatty acid ester of sucrose (equivalent mixture of“S-570” and “S-770” manufactured by Mitsubishi-Kagaku Foods Corp.) and490.2 g of cyclohexane were charged into a 2-liter beaker and the fattyacid ester of sucrose was dissolved by heating under stirring. Then thissolution was added to a flask with a capacity of 2 liters equipped witha stirring mechanism consisting of a pitched paddle stirring blade andtwo vertical baffles. The above-mentioned aqueous solution of monomerwas added to this and was stirred at 600 rpm for one hour. Thus, thewater-in-oil type dispersion liquid was prepared (reaction liquid A).

On the other hand, another reactor having a capacity of 2 liters wasprepared which has a stirring mechanism consisting of a pitched paddlestirring blade and two vertical baffles, and further a thermometer, areflux condenser, and a nitrogen introduction tube. 5 g of fatty acidester of sucrose and 490.2 g of cyclohexane were added thereto. Thefatty acid ester of sucrose (equivalent mixture of “S-570” and “S-770”manufactured by Mitsubishi-Kagaku Foods Corp.) was dissolved by heatingto a temperature of 80° C. under stirring, and the solution temperaturewas kept at 80° C. while the internal atmosphere was replaced withnitrogen (reaction liquid B).

While stirring both the reaction liquid A and the reaction liquid B at arevolution of 600 rpm, the reaction liquid A was dropped to the reactionliquid B, thereby performing polymerization. The whole amount of thereaction liquid A was planned to be dropped in one hour, but attachmentof a resin to the baffles and the inner wall of the reactor came to beobserved since the time when about 20 minutes had passed since the startof the dropping, and the growth of the attached gel with lapse of timewas observed.

After the completion of dropping the aqueous solution of monomer,heating and stirring were continued for 60 minutes. When the resultingin-oil dispersion liquid T-6 was discharged from the reactor, filtrationwas tried using a filter having a mesh opening of 75 μm, but cloggingoccurred immediately and the liquid was not able to be filtered. Whenthe in-oil dispersion liquid T-6 was taken out and observed, it wasfound that most part thereof was a huge aggregated mass and desiredmicroparticles were not obtained.

Comparative Example 2 Production of Polymer Microparticles T-9

Production was conducted in the same manner as that in Example 1, exceptthat a n-heptane solution of “Percumyl H80” was added from a hopper at atime when 1 hour and 40 minutes have passed since the charge of amonomer mixture, and that three minutes later an aqueous solution ofsodium hydrosulfite was fed in 30 seconds using a pump through anitrogen introduction tube.

The temperature of the contents in the reactor was increased immediatelyafter the time when the feed of the aqueous solution of sodiumhydrosulfite was started and the temperature of the reaction liquid wasreached 40° C. in about two minutes. At that time, large gel blocks werefound visually on the reaction liquid level. The results were shown inTable 1.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Polymer T-1 T-2 T-3 T-4microparticles Crosslink density 2.2 2.2 2.2 2.2 (mol %) Sorbitanmonooleate 2.0 2.0 2.0 2.0 UM-1HP 1.3 1.3 1.3 1.3 (pure content) Fattyacid ester — — — — of sucrose Order of Reducing agent Reducing agentReducing agent Reducing agent catalyst addition (Hopper/at one time)(Hopper/at one time) (Hopper/at one time) (Hopper/at one time) ↓ ↓ ↓ ↓(Feed port/Feed time) Oxidizing agent Oxidizing agent Oxidizing agentOxidizing agent (Side wall/30 sec) (Side wall/10 sec) (Side wall/120sec) (Side wall/180 sec) Condition of ◯ ◯ ◯ ◯ attachment on wallFilterability of ◯ Δ ◯ ◯ polymerization slurry Swollen particle 38.939.2 40.3 41.2 diameter (μm) Water absorption 20.4 20.1 23.1 21.5 ratio(times) Wet sieve residue 0.004 0.087 0.055 0.329 (%, after dehydration)Comparative Comparative Example 5 Example 6 Example 7 Example 1 Example2 Polymer T-5 T-6 T-7 T-8 T-9 microparticles Crosslink density 2.2 2.22.2 2.2 2.2 (mol %) Sorbitan monooleate 2.0 3.4 2.0 — 2.0 UM-1HP 1.3 0.01.3 — 1.3 (pure content) Fatty acid ester — — — 5.0 — of sucrose Orderof Reducing agent Reducing agent Reducing agent — Oxidizing agentcatalyst addition (Hopper/at one time) (Hopper/at one time) (Hopper/atone time) (Hopper/at one time) ↓ ↓ ↓ ↓ (Feed port/Feed time) Oxidizingagent Oxidizing agent Oxidizing agent Reducing agent (Top/30 sec) (Sidewall/30 sec) (PBH) (Side wall/30 sec) (Side wall/30 sec) Condition of ◯Δ ◯ X X attachment on wall Filterability of ◯ Δ ◯ X X polymerizationslurry Swollen particle 40.5 40.2 39.7 — 46.9 diameter (μm) Waterabsorption 22.1 20.3 20.8 — 25.1 ratio (times) Wet sieve residue 0.0080.097 0.039 — 0.468 (%, after dehydration)Referential Test: Check of Aptitude for Increasing Scale

The results of tests performed in Examples and Comparative Examplesusing a 250-liter reactor (a 2-liter reactor for T-8 only) are provided.Moreover, the results of tests performed by changing the productionscale under the conditions of Examples 1 to 7 and Comparative Examples 1to 2 are provided in the following Table 2. The evaluation conditionsgiven in Table 2 are the same as those of Table 1 above.

TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 Polymer T-1T-2 T-3 T-4 T-5 microparticles Crosslink density 2.2  2.2  2.2  2.2 2.2  (mol %)  2 L Condition of ◯ ◯ ◯ ◯ ◯ attachment on wallFilterability of ◯ ◯ ◯ ◯ ◯ polymerization slurry Wet sieve residue 0.0020.018 0.022 0.120 0.002 (%, after dehydration) 250 L Condition of ◯ ◯ ◯◯ ◯ attachment on wall Filterability of ◯ Δ ◯ ◯ ◯ polymerization slurryWet sieve residue 0.004 0.087 0.055 0.329 0.008 (%, after dehydration) 5 m³ Condition of ◯ Δ ◯ ◯ ◯ attachment on wall Filterability of ◯ Δ ◯ ◯◯ polymerization slurry Wet sieve residue 0.004 0.375 0.090 1.76  0.071(%, after dehydration) Comparative Comparative Example 6 Example 7Example 1 Example 2 Polymer T-6 T-7 T-8 T-9 microparticles Crosslinkdensity 2.2  2.2  2.2 2.2  (mol %)  2 L Condition of ◯ ◯ X ◯ attachmenton wall Filterability of ◯ ◯ X ◯ polymerization slurry Wet sieve residue0.034 0.031 Unmeasurable 0.065 (%, after dehydration) 250 L Condition ofΔ ◯ Unperformable X attachment on wall Filterability of Δ ◯ Xpolymerization slurry Wet sieve residue 0.097 0.039 0.468 (%, afterdehydration)  5 m³ Condition of Δ ◯ Unperformable Unperformableattachment on wall Filterability of Δ ◯ polymerization slurry Wet sieveresidue 0.217 0.153 (%, after dehydration)

The polymer microparticles analysis conditions (1) to (4) used inExamples are as follows.

(1) Solid Concentration

The weight (a) of about 1 g of a sample was measured, the weight (b) ofa residue after drying in a windless oven at a temperature of 150° C.for 60 minutes was measured, and then the solid concentration wascalculated by the following equation. For the measurement a weighingbottle was used. Other operations were performed in accordance with JISK0067-1992 (Test methods for loss and residue of chemical products).Solid concentration(%)=(b/a)×100(2) Water Absorption Ratio

The water absorption ratio was measured according to the followingmethod. The measuring device is illustrated in FIG. 3.

The measuring device is composed of <1> to <3> shown in FIG. 3.

<1> is consisting of a burette 1 having a branch pipe for airventilation, a pinch cock 2, a silicone tube 3, and apolytetrafluoroethylene tube 4.

In <2>, a supporting cylinder 8 having many holes in its bottom ismounted on a funnel 5, and a filter paper 10 for device is mountedthereon.

In <3>, a sample 6 of the polymer microparticles is inserted into twofilter papers 7 for fixing sample, and the filter papers for fixingsample are fixed with an adhesive tape 9. All the filter papers to beused are “ADVANTEC No. 2” having an inner diameter of 55 mm.

<1> and <2> are linked with the silicone tube 3.

The levels with respect to the burette 1 of the funnel 5 and thesupporting cylinder 8 are fixed, and the lower end of thepolytetrafluoroethylene tube 4 disposed within the burette branch pipeand the bottom of the supporting cylinder 8 are set to be at the samelevel (dotted line in FIG. 3).

The measuring method is described below.

The pinch cock 2 in <1> was released, and ion exchange water was chargedfrom the top of the burette 1 through the silicone tube 3 so that thespace from the burette 1 to the filter paper 10 for device was filledwith ion exchange water 12. Subsequently, the pinch cock 2 was closedand air was removed through the polytetrafluoroethylene tube 4 connectedto the burette branch pipe with a rubber stopper. Thus, a condition wasobtained such that ion exchange water 12 was continuously fed from theburette 1 to the filter paper 10 for device.

After that, excess ion exchange water 12 which oozed from the filterpaper 10 for device was removed, and then a read graduation (a) of theburette 1 was recorded.

A dry powder was sampled in an amount of 0.1 to 0.2 g, and then thepowder was placed uniformly on the center of the filter paper 7 forfixing sample as illustrated in <3>. Another filter paper was used tosandwich the sample and the two filter papers were adhered with anadhesive tape 9 to fix the sample. The filter papers between which thesample was fixed were put on the filter paper 10 for device asillustrated in <2>.

Subsequently, a read graduation (b) of the burette 1 after a lapse of 30minutes from a time when a lid 11 was put on the filter paper 10 fordevice was recorded.

The total (c) of the water absorption of the sample and the waterabsorption of the two filter papers 7 for fixing sample was calculatedby (a-b). By the same operation, the water absorption (d) of only thetwo filter papers 7 containing no water-absorptive polymer sample wasmeasured.

The above-mentioned operations were performed and a water absorptionratio was calculated from the following equation. As to the solidconcentration to be used for the calculation, a value measured by themethod (1) was used.

Water absorption ratio (times)=(c-d)/{Weight of sample (g)×(Solidconcentration (%)/100)}+100/(Solid concentration (%))

(3) Water-Swollen Particle Size

To 0.02 g of a sample for measurement was added 20 ml of ion exchangewater, followed by shaking well. Thus, the sample was disperseduniformly. For a dispersion liquid resulting from dispersion continuedfor 30 minutes or more in order to bring the polymer microparticles intoa water-saturated-swollen state, the particle size distribution wasmeasured after one-minute application of ultrasonic wave by using alaser diffraction scattering type particle size distribution analyzer(“MT-3000” manufactured by NIKKISO CO., LTD.). Ion exchange water wasused as a circulated dispersion medium used in the measurement and therefractive index of the dispersion was adjusted to 1.53. The mediandiameter (μm) was calculated from the particle size distribution onvolume basis obtained by the measurement, and it was defined as awater-swollen particle size.

(4) Measurement of Amount of Particle Having Water-Swollen Particle Sizeof 150 μm or Larger (a Wet Sieving Residue Method)

Measurement was performed in accordance with JIS K 0069-1992 (testmethod for sieving of chemical products).

A sample in an amount corresponding to 50 g in terms of solidconcentration was weighed and ethanol was added thereto in the sameamount as the sample to loosen well. Then the liquid was poured slowlyinto 3.0 liters of ion exchange water under stirring and stirred for 30minutes to prepare a water-swollen dispersion liquid of the sample.After confirmation of the uniform dispersion, the dispersion liquid waspoured onto a sieve having a diameter of 70 mm and a mesh opening of 150μm and allowed to pass therethrough. The residue on the sieve was washedwith a sufficient amount of water while taking care that the residuedoes not spill off. Subsequently, the sieve after measurement was driedin a circulation dryer at a temperature of 150° C. for 30 minutes andcooled in a desiccator, and then the weight of the sieve after drying(the total weight of the sieve and the residue) was measured.

The wet sieve residue (%) calculated by the following formula wasdefined as the amount of particle having a water-swollen particle sizeof 150 μm or larger. Operations other than those described above wereperformed in accordance with JIS K 0069-1992 (test method for sieving ofchemical products).Wet sieve residue(%)=[(Weight of sieve after test−Weight ofsieve)/{(Weight of sample used×(Solid concentration/100))}]×100

The results described above showed that no resin attachment to the innerwall of the reactor caused by a polymerization reaction occurred and thefilterability of the polymerization slurries was good in the productionmethod of the present invention. Therefore, it was found that a particleuniform in particle size could be produced in a good productivitywithout the occurrence of particle aggregation. On the other hand, itwas found that in Comparative Examples 1 and 2, which are conventionaltechnologies, the productivity was poor, and particularly on a practicalproduction scale it was impossible to produce a particle uniform inparticle size.

INDUSTRIAL APPLICABILITY

According to the production method of the present invention,high-quality spherical hydrophilic polymer microparticles remarkablyhigher uniform in particle size than those by conventional technologiescan be produced at high productivity while keeping high dispersionstability and polymerization stability without causing aggregation,clumping and adhesion to a polymerization apparatus of particles duringor after polymerization. Moreover, according to the production method ofthe present invention, even if a particle having a high degree ofcrosslinking is produced using a large amount of a multifunctionalvinyl-based monomer, high-quality hydrophilic crosslinked polymermicroparticles uniform in particle size can be produced without causingaggregation, clumping and adhesion to a polymerization apparatus ofparticles.

1. A method for producing polymer microparticles by inverse suspensionpolymerization of a vinyl-based monomer, wherein an oil-solubleoxidizing agent and a water-soluble reducing agent are used as apolymerization initiator, and wherein said oil-soluble oxidizing agentis fed after said water-soluble reducing agent is fed.
 2. The method forproducing polymer microparticles according to claim 1, wherein all ofsaid oil-soluble oxidizing agent is fed over a time range from 20seconds to 120 seconds.
 3. The method for producing polymermicroparticles according to claim 1, wherein said oil-soluble oxidizingagent is fed to a reactor through a feed port located below the reactionliquid level.
 4. The method for producing polymer microparticlesaccording to claim 1, wherein a macromonomer having a radicallypolymerizable unsaturated group at an end of a polymer derived from avinyl-based monomer is used as a dispersion stabilizer.
 5. The methodfor producing polymer microparticles according to claim 1, wherein saidpolymer microparticles produced by inverse suspension polymerization arepolymer microparticles having a crosslink density of 0.5% or more bymol.
 6. The method for producing polymer microparticles according toclaim 1, wherein said polymer microparticles produced by inversesuspension polymerization are polymer microparticles which have anaverage particle size in a saturated water-swollen state of 2 to 100 μm,and a content ratio of particles having a particle size of 150 μm orlarger in a saturated water-swollen state of 1.0% or less by weight. 7.The method for producing polymer microparticles according to claim 1,wherein said polymer microparticles produced by inverse suspensionpolymerization are polymer microparticles which have a water absorptionratio of 5 to 50 times, an average particle size in a saturatedwater-swollen state of 5 to 70 μm, and a content ratio of particleshaving a particle size of 150 μm or larger in a saturated water-swollenstate of 0.3% or less by weight.